Yellow fever was among the most feared diseases in nineteenth-century Europe. Occurrences on the Continent were rare yet the threat they posed provoked extraordinary medical concern and investigation. The disease played an important part in the prolonged struggle between the rival theories of contagionism and noncontagionism, and the renewed success of the former after 1860 had a role in winning wide acceptance for the germ-theory of disease. I shall examine the arguments and, above all, their relation to available evidence that once again brought yellow fever under the banner of contagionism; my principal but not exclusive focus will be medical discussion of the epidemic at Saint-Nazaire in 1861. I view this study as the introduction to a larger undertaking, namely, a history of the creation of an epidemiological outlook, partner with the clinic and laboratory in the scientific study of disease. It is an outlook which is also intrinsically social and has thus rendered and today renders the study of communicable disease an essential element in empirical social science.